Images may be formed and developed on the surface of photoconductive and insulating materials by electrostatic methods. An electrostatic latent charged image is formed on an insulating electrostatographic element and the latent image is rendered visible by a development step, wherein the latent image element is brought into contact with a developer mixture. In electrophotography, a photoconductor is charged and then exposed imagewise to light. In the area of the photoconductor exposed to light, the charge dissipates or decays while the dark areas retain the electrostatic charge. The resultant latent electrostatic image on the photoconductor may be developed by depositing toner particles over the surface of the photoconductor with the toner particles having a charge so as to be directed by the electrical fields to the image areas of the photoconductor to develop the electrostatic image, suitably biased to deposit toner on the discharged areas of the photoconductor. Subsequently, the toner image can be transferred to a support surface such as paper where it can be permanently affixed to the support surface using a variety of techniques including pressure fixing, heat fixing, solvent fixing and the like.
Developer material, comprising relatively large carrier particles having finely divided toner particles electrostatically clinging to the surface of the carrier particles, is conveyed to and contacted with the electrostatic latent image bearing surface. The toner particles are attracted to the electrostatic latent image by electrostatic attraction.
Carrier materials used in the development of electrostatic latent images are described in many patents including, for example, U.S. Pat. No. 3,590,000 (Palermiti et al.). The type of carrier material used depends on many factors such as the type of developer used, the quality of the development desired, the type of photoconductive material employed, and the like. Generally, carrier particles or the coating thereon should have a triboelectric value commensurate with the triboelectric value of the toner in order to generate electrostatic adhesion of the toner to the carrier. The toner and carrier particles of the developer material are selected so that the toner obtains the correct charge polarity and magnitude to insure that the toner particles are preferentially attracted to the desired image areas of the photoconductor. If the triboelectric charge is too low, the copy will be characterized by high print density but heavy background; if the charge is too high, the background is good but the print density will tend to be low. Thus, there is an optimum range of toner charge for best overall results.
Some dry developer materials which are employed in automatic copying machines, have carrier filming problems, due to the recycling of the carrier particles through many cycles producing many collisions between the carrier particles and between the carrier particles and parts of the machine. The attendant mechanical friction causes some toner material to form a physically adherent film on the surfaces of the carrier particles which impairs the normal triboelectric charging of the toner particles in the developer mix, resulting in a less highly charged toner. The improperly charged toner particles can be deposited on non-image areas, impairing the quality of the copies.
When toner filming occurs to a sufficient degree, the entire developer material must be replaced, increasing the cost of the operation of the machine. Furthermore, because of the contact between the carrier particles and between the carrier particles and parts of the machine, there is abrasion of the coating of the carrier particles. Even if the coating of the carrier particle resists abrasion, the coating must have good adhesion to the core of the carrier particle; otherwise, the coating can chip, flake, or spall, requiring early replacement of the developer material. This abrasion and wearing of the coating also may reduce the effectiveness of the triboelectric charging between the carrier and the toner by exposing the toner to the core material of the carrier.
Therefore, in addition to having the proper triboelectric characteristics, the coating of a carrier particle must have good anti-stick (low surface energy) properties to prevent filming of the carrier particle by the toner, good adherence to the core and be resistant to abrasion. Fluoropolymers such as fluorocarbons and fluorosilicones, for example, are materials having good anti-stick properties to prevent or greatly inhibit toner filming thereon as well as being capable of adhering to a core and resisting abrasion. It has previously been suggested in U.S. Pat. No. 3,533,835 (Hagenbach et al.) to employ fluorocarbons such as polytetrafluoroethylene as a coating for a carrier particle if finely-divided conductive particles are impacted into the coating. However, polytetrafluoroethylene has been described as being at or near the bottom of any published triboelectric series.
U.S. Pat. No. 3,873,356 (Queener et al.) discloses a method of coating a mixture of fluorocarbon and a modifying material on a carrier core particle so that the carrier core particle has the characteristic of being triboelectrically positive with respect to many toners. The modifying resin in which the fluoropolymer is essentially insoluble may be an epoxy resin, a urethane resin, or a methyl phenyl silicone resin. Because of the fluorocarbon in the mixture, the coating of the carrier particle has desired properties of resistance to abrasion, adherence to the core, and an antistick surface so that the filmed layer of toner cannot form thereon while still having the characteristic of being triboelectrically positive with respect to various toners. This is achieved by heating the coated carrier particles at a temperature at which the coating adheres to the core and becomes triboelectrically positive with respect to various toners. The coating may be applied to the core by any suitable means such as dipping, spraying, tumbling the cores with a coating solution in a barrel, or through a fluidized bed.
U.S. Pat. No. 4,233,387 (Mammino et al.) discloses dry mixing of carrier particles with thermoplastic resin particles until the thermoplastic resin particles adhere to the carrier core particles by mechanical impaction and/or electrostatic attraction. The dry mixture is then heated to a temperature of between 320.degree. F. and about 650.degree. F. for between 120 minutes and about 20 minutes so that the thermoplastic resin particles melt and fuse to the carrier core particles. After fusion of the resin particles to the carrier core particles, the coated carrier particles are cooled and classified to the desired particles size. The resultant coated carrier particles have a fused resin coating over between about 15 percent and up to about 85 percent of their surface area.
U.S. Pat. No. 4,209,550 (Hagenbach et al.) discloses a method of coating carrier materials by electrostatically attracting particles of a coating material to the surface of carrier cores and then heating the carrier materials, causing the coating material to fuse to the carrier material forming an adherent coating thereon.
Materials which may be used to coat the carrier core particles include but are not limited to polyvinyl fluoride, polyvinyl chloride, polyvinylidene fluoride, polyvinylidene chloride, homopolymers and copolymers of other vinyls such as vinyl chloride and trifluorochloroethylene, copolymers of vinylidene fluoride and tetrafluoroethylene, copolymers of vinylidene fluoride and hexafluoropropylene, and terpolymers of, for example, vinylidene fluoride and hexafloropropylene and tetrafluoroethylene. These materials may be attached to carrier core particles by melting the coating material and fusing it to the carrier core particles. The adhesion of the carrier coating on the core depends, in large measure, on the melt rheology of the polymer, the dwell time that the carrier cores and the coating particles or resins are in the furnace and the temperature of the furnace. For example, polyvinylidene fluoride (PVF.sub.2), available as Kynar.RTM. from Pennwalt Corporation, may be heated from about 190.degree. C. to about 265.degree. C. with good melt rheology, but quickly discolors if overheated. Thermal decomposition occurs at about 375.degree. C., releasing toxic anhydrous hydrogen fluoride gas. Polyvinyl fluoride (PVF), sold by DuPont under the trademark Tedlar.RTM., melts at about 190.degree. C. and starts to decompose at about 210.degree. C., also liberating hydrogen fluoride gas. The thermal processing latitude for PVF is less than for PVF.sub.2. Coating with PVF is further complicated in that PVF is not soluble in substantially any solvent at room temperature.